Apparatus for controlling boiler system

ABSTRACT

The output vapor pressure of a boiler system is controlled by selectively starting or stopping a plurality of boiler units (#1 to #4) shiftable between a high combustion state to a low combustion state in accordance with the output vapor pressure of the entire system or by shifting the boiler units between the high and low combustion states. At least two boiler units among the plurality of boiler units are selected as adjustment units in accordance with the output vapor pressure of the entire system. Each of the adjustment units is forced to shift alternately between the high and low combustion states in accordance with the output vapor pressure while continuously holding the adjustment units in the operating state.

TECHNICAL FIELD

The present invention generally relates to a heating system comprising aplurality of heating units, and more particularly, to the heating systemequipped with a plurality of heating units shiftable from a highoutputting state to a low outputting state and vice versa during itsoperation.

As an example of such a heating system, there is typically a boilersystem comprising a plurality of boiler units. Hence, the presentinvention deals with the boiler system provided with a plurality ofboiler units shiftable from a high combustion state to a low combustionstate and vice versa during its operation.

BACKGROUND OF THE ART

The boiler system formed of a plurality of boilers each having arelatively small capacity exhibits a variety of advantages as comparedwith a system which employs a single boiler of a large capacity. To bespecific, there is generally almost no chance for the large capacityboiler to be operated under a condition of the maximum output. It istherefore quite rare that its potential evaporation capability iseffectively utilized. In sharp contrast with this, the boiler systembased on combinations of small capacity boilers is arranged in such away that in the great majority of cases the individual boilers functionat their maximum output, and it follows that the potential capabilitiesthereof are efficiently exhibited. This is the most of the advantagesmentioned above.

In general, the boiler is inevitably required to go through thepreparatory stage which is commonly referred to as "prepurge"(ventilating or scavenging) and a subsequent low output state during aprocess originating from the halting state to a high output state (highcombustion state), irrespective of the magnitude of capacity thereof.The preparatory stage causes not a little delay in time when startingthe boilers. For this reason, it is desirable to remain the boilerswhich have once started in the continuous combustion state to thegreatest possible degree, thereby causing the time lag at the startingtime to be minimized, and making the boilers follow fluctuations in load(the amount of evaporation required per unit time) on the system quicklyand smoothly.

For the purpose of satisfying the above-mentioned requirements, a widevariety of systems for controlling the boiler system have heretoforebeen proposed. Among conventional control systems, the arrangement of awell-known control system is such that the respective boiler units whichare combined to constitute a boiler system are stepwise controlled at,e.g., two stages--i.e., these boiler units are shifted alternatelybetween the high and low output states, or, that the boiler units areconsecutively controlled to vary the outputs thereof with nointerruption between the predetermined maximum and minimum values. Atypical system in the former case is disclosed in, e.g., Japanese PatentPublic Disclosure No. 81401/79, while a system in the latter case isdisclosed in, e.g., U.S. Pat. No. 3,387,589.

In all these known systems for controlling the boiler system, a specificboiler unit among the plurality of boiler units, viz., the boiler unitwhich starts lastly, is operated to shift alternately from the lowoutput state to the high output state in accordance with fluctuations inload, such a boiler unit is controlled to be brought into the haltingstate (a stepwise control system), or the boiler unit which startslastly is controlled to shift alternately from the operating state tothe halting state between the maximum and minimum output values (acontinuous control system). In either case, the specific boiler unit isforced to frequently start and stop. As discussed above, the startingfrom the halting state always involves the above-mentioned prepurge andsubsequent low output state. This is one of the factors whichoutstandingly degrade a response of the boiler system to thefluctuations in load.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention which eliminatesthe above-described disadvantageous factors to provide an apparatus forcontrolling a boiler system wherein at least two boiler units areconstantly secured as adjustment units in a stationary state of theboiler system, and wherein these two or more boiler units are eachcontrolled to properly shift from a high output state to a low outputstate and vice versa, viz., between a high combustion state and a lowcombustion state in accordance with variations in the output vaporpressure, i.e., variations in the entire output of the boiler system, byconstantly maintaining the adjustment units to be in the combustionstate, thereby causing the output of the entire boiler system tosmoothly converge in a desired range without repeatedly stopping andstarting each boiler unit.

To this end, the present invention is intended to provide an apparatusfor controlling a boiler system, comprising: a plurality of boiler units10 connected in common to a steam header 11 and each shiftable between ahalting state and an operating state in response to each of a combustionstarting signal BI and a combustion stopping signal BD, and furthershiftable, in the operating state, between a high combustion state and alow combustion state in response to each of a high combustion stateselecting signal BH and a low combustion state selecting signal BL; apressure adjuster 12 connected to the steam header 11 for outputting avapor pressure signal P indicating the vapor pressure in the header; anda control unit 13 for controlling each of the plurality of boiler unitsin the three states in response to the vapor pressure signal Ptransmitted from the pressure adjuster 12. This control unit 13includes: combustion unit number increase request signal generatingmeans 15a for outputting a combustion unit number increase requestsignal Bi by detecting the fact that the vapor pressure indicated by thevapor pressure signal P is equal to or smaller than a first criticalvalue; high combustion state request signal generating means 15b foroutputting a high combustion state request signal Bh by detecting thefact that the vapor pressure indicated by the vapor pressure signal P isequal to or smaller than a second critical value which is greater thanthe first critical value; low combustion state request signal generatingmeans 15c for outputting a low combustion state request signal Bl bydetecting the fact that the vapor pressure indicated by the vaporpressure signal P is equal to or larger than a third critical valuewhich is greater than the second critical value; combustion unit numberdecrease request signal generating means 15d for outputting a combustionunit number decrease request signal Bd by detecting the fact that thevapor pressure indicated by the vapor pressure signal P is equal to orlarger than a fourth critical value which is greater than the thirdcritical value; combustion unit number setting means 17 for sequentiallysupplying each of the plurality of boiler units 10 with the combustionstarting signal BI in response to the combustion unit number increaserequest signal Bi according to the order of starting the boiler unitsevery time a predetermined period passes and for sequentially supplyingeach of the plurality of boiler units with the combustion stoppingsignal BD in response to the combustion unit number decrease requestsignal Bd in the order reverse to the starting order every time apredetermined period passes; combustion state selecting means 18 forsequentially supplying each of the plurality of boiler units 10 with thehigh combustion state selecting signal BH in response to the highcombustion state request signal Bh in the order of starting the boilerunits every time a predetermined period passes and for sequentiallysupplying each of the plurality of boiler units 10 with the lowcombustion state selecting signal BL in response to the low combustionstate request signal Bl in the order reverse to the starting order everytime a predetermined period passes; adjustment unit number setting means21 for outputting an adjustment unit number increase signal Bp bydeciding that the number of boiler units which are operating in the lowcombustion state is smaller than the number of adjustment units on thebasis of the combustion starting signal BI supplied from the combustionunit number setting means 17 to each of the plurality of boiler units10, the low combustion state selecting signal BL supplied from thecombustion state selecting means 18 to each of the plurality of boilerunits 10 and an adjustment unit number setting signal indicating thenumber of adjustment units; and low combustion state request signalsupplying means 19 for allowing the low combustion state request signalBl to be supplied to the combustion state selecting means 18 in responseto the adjustment unit number increase signal Bp.

In accordance with the present invention, at least two boiler units maybe constantly kept in the combustion state in the stationary state ofthe boiler system, and these two or more boiler units may be eachcontrolled to shift between the high and low combustion states inaccordance with variations in the output of the entire boiler system,viz., variations in the output vapor pressure, thereby minimizing thefrequency at which the respective boiler units are repeatedly stoppedand started. As a result, it is possible to obtain an effect ofpermitting the output of the entire boiler system to smoothly convergein a desired range. Besides, the larger the number of adjustment unitsis, the more the number of boiler units which shift from the highcombustion state to the low combustion state and vice versa increases.Hence, it is also possible to obtain an advantage of being able to copewith the variations in load with no precharge sequent to the startingprocess, that is, without increasing the number of combustion units whenshifting to a higher load.

In an embodiment of the present invention, if a sharp fluctuation in thevapor pressure takes place a drop or a rise in the pressure within theboiler system is restrained while sequentially shifting the adjustmentunits from the low combustion state to the high combustion state andvice versa at 3-second intervals. Nevertheless, if the pressure isfurther increased or decreased to enter a control region A or E, thenumber of boiler units in combustion state is gradually increased orreduced. The larger the number of set adjustment units is, the largerthe number of boiler units shifting alternately between the lowcombustion state and the high combustion state with the delay of 3 secbecomes. It is therefore possible to effectively cope with fluctuationsin load with no increment in the number of boiler units in combustionstate when, e.g., shifting to a higher load.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram schematically illustrating a boiler systemto which a boiler controlling apparatus according to the presentinvention is applicable;

FIGs. 2A and 2B are a block diagram conceptually showing an example ofconstitution of a hardware for executing algorithm of the controllingapparatus according to the present invention, as a concrete example ofconstitution of the controlling apparatus in the system depicted in FIG.1;

FIG. 3 is a block circuit diagram illustrating one example ofconstitution of a circuit of an adjustment unit number setting sectionin the controlling apparatus depicted in FIG. 2; and

FIG. 4 shows a curved line of vapor pressure when the boiler controllingapparatus according to the present invention is applied to the boilersystem depicted in FIG. 1, logical states of control signals selectivelysupplied to the system at respective points of time on the curved line,and a time chart which shows outputting states of the respective boilerunits.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment when an apparatus for controlling a heating systemaccording to the present invention is applied to the control of outputof a boiler system having a plurality of boiler units will hereinafterbe described with reference to the accompanying drawings. It is to benoted that the boiler system to which the controlling apparatusaccording to the present invention can be applied may include any typeof and any number of boiler units. In the embodiment that will bedescribed below, however, for convenience of description, the boilersystem is constituted by connecting four units of steam boilers.

FIG. 1 schematically illustrates a boiler system 10 to which thecontrolling apparatus according to the present invention may be applied.The boiler system 10 includes, as explained earlier, four steam boilerunits (hereinafter simply referred to as boiler units). These fourboiler units are each marked with the symbols #1, #2, #3 and #4 inaccordance with the order of starting these boiler units. Outputs of theboiler units #1 to #4 are connected in common to a steam header 11 andare further led therethrough to a load device (not shown). The steamheader 11 is provided with a pressure adjuster 12 by which the vaporpressure in the steam header is always adjusted to a substantiallypredetermined value. The pressure adjuster having such a function iswell known and the detailed description is therefore omitted herein.

The boiler unit controlling apparatus according to the present inventionselectively controls the boiler units #1 to #4, that are combined toform the boiler system 10, by use of a control unit 13 employing amicrocomputer on the basis of the vapor pressure in the steam header 11which is detected by a variable resistor incorporated into the pressureadjuster 12.

FIG. 2 conceptually shows a hardware constitution for executingalgorithm of the controlling apparatus, as a concrete example ofconstitution of the above-mentioned control unit 13. According to theillustrated hardware constitution, the resistance value of the variableresistor incorporated into the pressure adjuster 12 is converted intothe voltage value by means of a resistance-voltage converting circuit14, which outputs the voltage proportional or corresponding to the vaporpressure in the steam header 11 which is detected by the pressureadjuster 12. The output voltage of the resistance-voltage convertingcircuit 14 is input to a voltage comparing circuit 15 comprising aplurality of comparators, i.e., four comparators 15a through 15d in thisembodiment. In each of these four comparators 15a through 15d, thereference voltage value corresponding to the critical value of thepredetermined vapor pressure in the steam header 12 that is detected bythe pressure adjuster 12 is set. When the voltage input to eachcomparator exceeds the reference voltage value, the voltage changing toa high or low level is output. In this embodiment, it is assumed that,for instance, the reference voltage value corresponding to the firstcritical value 7.2 kg/cm² is set in the first comparator 15a; thereference voltage value corresponding to the second critical value 7.4kg/cm² is set in the second comparator 15b; the reference voltage valuecorresponding to the third critical value 7.8 kg/cm² is set in the thirdcomparator 15c; and the reference voltage value corresponding to thefourth critical value 8.0 kg/cm² is set in the fourth comparator 15d.Outputs from the comparators 15a to 15d are input through, e.g., buffers16a to 16d to a processor of a microcomputer where these signals areprocessed.

More specifically, the output from the first comparator 15a is input asa combustion unit number increase request signal Bi through the buffer16a to a combustion unit number setting section 17, while the outputfrom the fourth comparator 15d is input as a combustion unit numberdecrease request signal Bd through the buffer 16d to the same settingsection 17. Furthermore, the output from the second comparator 15b isinput as a high combustion state request signal Bh through the buffer16b to a combustion state selecting section 18, and the output from thethird comparator 15c is input as a low combustion state request signalBl through the buffer 16c to one input of an AND gate 19. It is notedthat these output signals Bi, Bd, Bh and Bl are assumed to beactive-high signals and become active at the high level when the inputvoltage of the comparators 15a and 15b is equal to or smaller than therespective reference voltage and when the input voltage of thecomparators 15c and 15d is equal to or greater than the respectivereference voltage.

When a starting switch 20 is turned ON, the combustion unit numbersetting section 17 is activated, and a combustion starting signal BI atthe high level or a combustion stopping signal BD at the low level isoutput to be selectively supplied to the boiler units #1 to #4 inresponse to the input signal Bi or Bd. The output signal BI or BD fromthe combustion unit number setting section 17 is further input to anadjustment unit number setting section 21. During the period for whichthe combustion unit number increase request signal Bi is present in theinput, the combustion unit number setting section 17 sequentiallyoutputs the combustion starting signals BI at the high level to theboiler units at predetermined intervals in accordance with the order ofstarting the boiler units, viz., in the order #1→#2→#3→#4. The timeinterval at which the combustion starting signal BI at the high level issequentially supplied to the respective boiler units is preferablyaround 10 sec, taking into consideration some time lag produced when therespective boiler units are activated from their halting state and reachthe high combustion state. Where the boiler units each shift from thehalting state to the combustion state in response to the combustionstarting signal BI transmitted from the combustion unit number settingsection 17, it is presumed that the boiler units shift unconditionallyto the high combustion state. Prior to the shifting of the boiler unitsfrom the halting state to the high combustion state, there are more orless a delay in time and a low combustion state period subsequentthereto.

On the other hand, during the combustion unit number decrease requestsignal Bd exists, the combustion stopping signal BD at the low level issequentially output to be supplied to the boiler units at predeterminedintervals of, for example, 4 sec in the order reverse to the boiler unitstarting order, i.e., in the order #4→#3→#2→#1.

The adjustment unit number setting section 21 and the combustion stateselecting section 18 cooperate to select, among the boiler units thatare in the combustion state, boiler units the number of which is set byan adjustment unit number setting switch 22. The selected boiler unitsare designated as adjustment units. The outputs of the section 21 isinput to the other input of the AND gate 19. The logical AND output ofthe AND gate 19 is input to the combustion state selecting section 18.The combustion state selecting section 18 selectively outputs a highcombustion state selecting signal BH at the high level or a lowcombustion state selecting signal BL at the low level to supply thesesignals to the boiler units #1 to #4 in response to the logical ANDoutput and the high combustion state request signal Bh transmitted fromthe second comparator 15b. The output signal BH or BL from thecombustion state selecting section 18 is also input to the adjustmentunit number setting section 21.

The next description will now be focused on the role of the adjustmentunits in the controlling apparatus according to the present invention.The adjustment units in the present controlling apparatus are boilerunits controlled to shift alternately between the low combustion stateand the high combustion state in the stationary state in order to absorbfluctuations in load. The number of adjustment units can be setarbitrarily, i.e., in a programable manner by the adjustment unit numbersetting switch 22. In this embodiment, it is supposed that the number ofadjustment unit set by the adjustment unit number setting switch 22 istwo, and that the two boiler units started lastly among the boiler unitsin combustion state are selected and designated as the adjustment units.During all the boiler units #1 to #4 of the boiler system 10 in thisembodiment are in combustion state, the two boiler units #4 and #3started lastly among those boiler units are designated as the adjustmentunits by the adjustment unit number setting section 21. On the otherhand, during the boiler units #1 to #3 are in combustion state, the twoboiler units #3 and #2 started lastly among those boiler units arelikewise designated as the adjustment units by the number settingsection 21. Sequentially output to the thus designated adjustment unitsare the low combustion state selecting signal BL at the low level atpredetermined intervals of, e.g., 3 sec in the order reverse to thestarting order--viz., in the order #4→#3 or #3→#2 or #2→#1--during theperiod for which the vapor pressure is rising and the high level signalis present in the output of the AND gate 19. During the period for whichthe vapor pressure is falling and the high combustion state requestsignal Bh appears, the high combustion state selecting signal BH at thehigh level is sequentially output to be supplied to the adjustment unitsat the time concerned at predetermined intervals of, for instance, 3 secin the starting order of #1→#2 or #2→#3 or #3→#4.

The adjustment unit number setting switch 22 comprises, for example, adigital potentiometer or the like and is arranged to output a voltagesignal proportional to the number of adjustment units set. Theadjustment unit number setting section 21 receives, as input signals, asetting signal from the adjustment unit number setting switch 22 andoutput signals from the combustion unit number setting section 17 andfrom the combustion state selecting section 18, and compares the numberof boiler units which receive, as the input signal from the combustionunit number setting section 17, the combustion starting signal BI at thehigh level and, as the input signal from the combustion state selectingsection 18, the low combustion state selecting signal BL at the lowlevel, with the number of boiler units which is indicated by the outputsignal from the adjustment unit number setting switch 22. If the latter,i.e., the number of adjustment units which are set by the switch 22exceeds the former, viz., the number of boiler units that are beingcontrolled to work in the low combustion state, the section 21 decidesthat the number of adjustment units is small, and then supplies theadjustment unit number increase signal Bp at the high level to one inputof the AND gate 19.

FIG. 3 illustrates an example of a concrete hardware construction foracquiring a function of the adjustment unit number setting section 21.As illustrated in FIG. 3, the output signals BI or BD transmitted fromthe combustion unit number setting section 17 to the boiler units #1 to#4 are applied to one input of four AND gates 23a to 23d separatelyprovided. The output signals BH or BL transmitted from the combustionstate selecting section 18 to the boiler units #1 to #4 are appliedthrough separately provided inverters 24a to 24d to the other input ofthe AND gates 23a to 23d. The outputs of the respective AND gates 23a to23d are connected via a voltage adding circuit 25 to one input of acomparator 26, while the other input of this comparator 26 is connectedto the adjustment unit number setting switch 22. The output signals BIat the high level selectively transmitted from the combustion unitnumber setting section 17 to the boiler units #1 to #4 are properlyinput to the AND gates 23a to 23d. The output signals BL at the lowlevel selectively transmitted from the combustion state selectingsection 18 to boiler units #1 to #4 are inverted by the inverters 24athrough 24d and are then adequately input as high level signals to theAND gates 23a to 23d. The output signals from the AND gates 23a to 23dare added by the adding circuit 25, which generates a voltage signalindicating the number of boiler units which are controlled to operate inthe low combustion state, that is the number of boiler units whichreceive, as the input signals from the combustion unit number settingsection 17, the combustion starting signals BI at the high level and, asthe input signals from the combustion state selecting section 18, thelow combustion state selecting signals BL at the low level. The outputsignal from the voltage adding circuit 25 is applied to one inputterminal of the comparator 26, and is compared with the output signalapplied from the adjustment unit number setting switch 22 to the otherinput terminal of the comparator 26. If the number of adjustment unitsset by the adjustment unit number setting switch 22 is larger than thenumber of boiler units that are being controlled to operate in the lowcombustion state, the output signal at the high level is output from thecomparator 26 as the adjustment unit number increase signals Bp to beapplied to one input terminal of the AND gate 19.

The combustion state selecting section 18 receives the adjustment unitnumber increase signals Bp through the AND gate 19 to effect control sothat the number of boiler units supplied with the low combustion stateselecting signal BL does not exceed the number of adjustment units setby the adjustment unit number setting switch 22. It is assumed that theboiler unit that is started lastly among the boiler units on combustionat a certain point of time comes to halt combustion, and thattransiently only one adjustment unit operates. In such a case, when thevapor pressure rises up to the third critical value 7.8 kg/cm² and thelow combustion state request signal Bl is present in one input terminalof the AND gate 19, the condition of logical product in the AND gate 19is established by the adjustment unit number increase signals Bp appliedfrom the adjustment unit number setting section 21 to the other inputterminal of the AND gate 19; and the boiler unit which is startedsubsequent to the adjustment unit at that time is designated as a newadjustment unit by the high level output of the AND gate. It followsthat this new adjustment unit shifts from the high combustion state tothe low combustion state in response to the low combustion stateselecting signal BL output from the combustion state selecting section18.

The combustion state selecting section 18 causes the adjustment unitdesignated at that time to shift, as described above, from the highcombustion state to the low combustion state at intervals of 3 sec inthe order reverse to the starting order when the logical AND output fromthe AND gate 19 assumes high level. However, if the boiler units thatare started later among the adjustment units have already been put inthe high combustion state, when the output signal is given from theadjustment unit number setting section 21, the boiler unit of the twoadjustment units which is started earlier shifts from the highcombustion state to the low combustion state.

In such a hardware construction, the boiler system 10 (FIG. 1) iscontrolled on the basis of a control protocol relative to the following5-stage control regions.

Control Region A (Operating Unit Number Increase Mode)

When the vapor pressure in the steam header 11 (FIG. 1) is equal to orsmaller than the first critical value 7.2 kg/cm², this region is definedas a pressure control region A. In the control region A, the boilerunits are sequentially started one by one in the high combustion stateat intervals of 10 sec. In this case, the order of starting the boilerunits which shift from the halting state to the high combustion state is#1→#2→#3→#4.

Control Region B (High Combustion State Shifting Mode)

When the vapor pressure is equal to or exceeds the first critical value7.2 kg/cm² but is equal to or smaller than the second critical value 7.4kg/cm², this region is defined as a pressure control region B. In thecontrol region B, the adjustment units designated at the time concernedshift from the low combustion state to the high combustion state one byone at intervals of 3 sec. In this case, the order of shifting the twoadjustment units from the low combustion state to the high combustionstate is #1→#2, #2→#3 or #3→#4 in accordance with the starting order. Ifthe vapor pressure is going up, however, every adjustment unit hasalready been in the high combustion state. Therefore, this mode isexecuted only when the vapor pressure is decreasing and the adjustmentunits are all in the low combustion state.

Control Region C (State Holding Mode)

When the vapor pressure is equal to or greater than the second criticalvalue 7.4 kg/cm² but is equal to or smaller than the third criticalvalue 7.8 kg/cm², this region is defined as a pressure control region C.In the control region C, all the boiler units hold the state at the timewhen the vapor pressure rises and exceeds the second critical value, orwhen the vapor pressure falls and comes to the third critical value orless.

Control Region D (Low Combustion State Shifting Mode)

When the vapor pressure is equal to or greater than the third criticalvalue 7.8 kg/cm² but is equal to or less than the fourth critical value8.0 kg/cm², this is called a control pressure region D. In this controlregion D, the adjustment units are caused to shift from the highcombustion state to the low combustion state one by one at intervals of3 sec. In this case, the order of shifting the two adjustment units fromthe high combustion state to the low combustion state is opposite to thestarting order, viz., #4→#3, #3→#2 or #2→#1. If the vapor pressure isdecreasing, however, every adjustment unit has already been in the lowcombustion state. For this reason, this mode is executed only when thevapor pressure is rising and the adjustment units are all in the highcombustion state.

Control Region E (Combustion Unit Number Decrease Mode)

When the vapor pressure is equal to or exceeds the fourth critical value8.0 kg/cm², this is called a pressure control region E. In this controlregion E, the number of the boiler units is reduced one by one atintervals of 4 sec, and simultaneously the adjustment units are made toshift from the high combustion state to the low combustion state one byone. In this case, the order of halting the boiler units is reverse tothe order of starting them, i.e., #4→#3→#2→#1. The order of shifting theadjustment units from the high combustion state to the low combustionstate is also opposite to the starting order, viz., #4→#3, #3→#2 or#2→#1.

Based on the above-mentioned control protocols, the operation ofcontrolling the boiler system having a construction depicted in FIG. 1will now be described with reference to FIGS. 2 and 4. In FIG. 4, thesymbols BH and BL indicate a high combustion state selecting signal anda low combustion state selecting signal from the combustion stateselecting section 18, respectively. The symbols H and L represent actualcombustion state of the boiler units--H indicates the high combustionstate, while L indicates the low combustion state--during the period forwhich the high and low combustion state selecting signals BH and BL areoutput. The low combustion state L of a short period prior to the risingof the high combustion state H of each boiler unit shows a lowcombustion period which inevitably exists subsequent to theabove-described prepurge. In the description that follows, however, itis assumed for the time being that the thermal load on the boiler systemis substantially constant. Then, a control operation when the loadfluctuates will be explained.

(1) To begin with, when the starting switch 20 depicted in FIG. 2 isturned ON at the time to in FIG. 4, the vapor pressure falls within theabove-mentioned pressure control region A. This control region A is, asdiscussed above, the region where the number of boiler units incombustion state increases one by one at intervals of 10 second. Becausethe outputs from the comparators 15a and 15b are held at the high level,the combustion unit number increase request signal Bi and the highcombustion state request signal Bh are already output. Accordingly, thecombustion starting signal BI from the combustion unit number settingsection 17 and the high combustion state selecting signal BH from thecombustion state selecting section 18 are output to be supplied to theboiler unit #1. As a result, the boiler unit #1 initiates combustion inthe high combustion state H after some prepurge has been effected andthe subsequent low combustion state period has passed, whereby the vaporpressure in the header 11 (FIG. 1) of the boiler system rises. At thetime t1 after 10 sec has passed since the boiler unit #1 initiatedcombustion, if the state included in the control region A stillcontinues, the combustion starting signal BI and the high combustionstate selecting signal BH are respectively further output from thecombustion unit number setting section 17 and the combustion stateselecting section 18 and are supplied to the boiler unit #2. The boilerunit #2 likewise initiates combustion in the high combustion state Hafter some prepurge has been effected and the subsequent low combustionstate period has passed. As far as the state in the control region Acontinues thereafter, the boiler unit #3 and other boiler units whichfollow sequentially start their combustion at intervals of 10 sec inaccordance with the starting order and are then brought into the highcombustion state. Thus, a plurality of boiler units have initiatedcombustion in the high combustion state. In the following description,it is presumed that the boiler units #3 and #4 are started at the timet2 and the time t3, respectively, and that all the boiler units #1through #4 of the boiler system concerned come into the high combustionstate. As explained earlier, if these boiler units #1 to #4 are all inthe combustion state, two boiler units #3 and #4 that are started laterare designated as the adjustment units.

(2) Since all the boiler units #1 to #4 initiate combustion in the highcombustion state in the above-mentioned manner, the vapor pressure inthe header immediately rises and comes to the first critical value 7.2kg/cm² at, e.g., the time t4 shown in the Figure. The pressure controlregion then moves from the region A to the region B, viz., to the highcombustion state shifting mode. This control region B is in such a modethat the adjustment units shift one by one from the low combustion stateto the high combustion state at 3-second intervals. In this embodiment,since the boiler unit #4 of the adjustment units #3 and #4 which isstarted later in the starting order is in the control region where theunit #4 is forced to shift from the low combustion state L to the highcombustion state H, and the output from the comparator 15b assumes thehigh level, the high combustion state request signal Bh is output. Theboiler unit #3 has already been in the high combustion state H, and thevapor pressure is increasing. Hence, the respective boiler units #1 to#4 remain as they are, viz., they are held in the high combustion stateH.

(3) When the vapor pressure further increases and reaches the secondcritical value 7.4 kg/cm² at, e.g., the time t5, the pressure controlregion shifts from the region B to the region C, i.e., to the stateholding mode. In this control region C, the outputs from the comparators15a to 15d all assume the low level. Hence, as far as the vapor pressurefalls within the range between the second critical value 7.4 kg/cm² andthe third critical value 7.8 kg/cm², all the boiler units hold theirstate, viz., the high combustion state H.

(4) Next, when the vapor pressure comes to the third critical value 7.8kg/cm² at, e.g., the time t6 shown in the Figure, the control regionchanges from the region C to the region D, i.e., to the low combustionstate shifting mode. This control region D is the region where theadjustment units shift one by one from the high combustion state to thelow combustion state at 3-second intervals. In this embodiment, thelevel of the output from the comparator 15c first becomes high, and thelow combustion state request signal Bl is then output. Therefore, thelow combustion state selecting signal BL is output from the combustionstate selecting section 18 and is supplied to the boiler unit #4 of theadjustment units #3 and #4 which is started later in the starting order,whereby this boiler unit #4 moves from the high combustion state H tothe low combustion state L. Three seconds after that the adjustment unit#3 that is next in order shifts similarly from the high combustion stateH to the low combustion state L at the time t7 in response to the lowcombustion state selecting signal BL transmitted from the combustionstate selecting section 18. However, if the vapor pressure continues torise and is still higher than the third critical value 7.8 kg/cm² at thetime t8 3 sec after the time t7, since the adjustment units #4 and #3 atthe present time are in the low combustion state L, the boiler units #1to #4 remain as they are.

(5) Thereafter, if the vapor pressure further increases and reaches thefourth critical value 8.0 kg/cm² at, e.g., the time t9 shown in theFigure, the control region shifts from region D to the region E. Thiscontrol region E is the region where the number of combustion units isreduced one by one at intervals of 4 sec, and at the same time theadjustment units are forced to shift one by one from the high combustionstate to the low combustion state. In this embodiment, the combustionstopping signal BD is output from the combustion unit number settingsection 17 to be supplied to the boiler unit #4, and simultaneously theoutputs from the comparators 15d and 15c assume the high level. Then,the low combustion state request signal Bd and the low combustion staterequest signal Bl are output. The AND gate 19 is simultaneously suppliedwith the low combustion state request signal Bl and the adjustment unitnumber increase request signal Bp from the adjustment unit numbersetting section 22, and then the low combustion state selecting signalBL is output from the combustion state selecting section 18. Thus, theboiler unit #4 that is operating in the low combustion state L haltscombustion, and concurrently the designation of adjustment units changesfrom the boiler units #4 and #3 to another set of boiler units #3 and#2. Since the boiler unit #3 of these adjustment units #3 and #2 whichis started later in the starting order has already been brought into thelow combustion state L at the time t7, the low combustion stateselecting signal BL is output from the combustion state selectingsection 18 and is supplied to the boiler unit #2 at the present time t9,making the boiler unit #2 shift to the low combustion state L. It isnoted that the combustion stopping signal BD to the boiler unit #4 andthe low combustion state selecting signal BL to the boiler unit #2 areoutput at a time. If the vapor pressure is yet greater than the fourthcritical value 8.0 kg/cm² at the time t10 4 sec after the time t9, thecombustion stopping signal BD is output from the combustion unit numbersetting section 17 to the boiler unit #3 at the time t10, whereby theboiler unit #3 stops combustion. At this time, the level of the outputfrom the comparator 15c becomes high, and the low combustion staterequest signal Bl is output. The thus output low combustion staterequest signal Bl and the adjustment unit number increase request signalBp from the adjustment unit number setting section 22 are simultaneouslyinput to the AND gate 19, with the result that the boiler unit #1 isnewly designated as the adjustment unit. The designation of adjustmentunits further changes from the boiler units #3 and #2 to boiler units #2and #1. Thus, the low combustion state selecting signal BL is outputfrom the combustion state selecting section 18 and is supplied to theboiler unit #1, thereby shifting the boiler unit #1 from the highcombustion state H to the low combustion state L. It is to be noted thatthe combustion stopping signal BD to the boiler unit #3 and the lowcombustion state selecting signal BL to the boiler unit #1 aresimultaneously output.

(6) Since the boiler units #4 and #3 stop combustion in theabove-mentioned manner, and the adjustment units #3 and #2 move to thelow combustion state L, the vapor pressure immediately drops down. Whenthe vapor pressure becomes smaller than the fourth critical value 8.0kg/cm² at, e.g., the time t11 shown in the Figure, the pressure controlregion changes from the region E to the region D. At that time, however,the adjustment units are already in the low combustion state. Therefore,the respective boiler units #2 and #1 in the low combustion state Lremain as they are.

(7) When the vapor pressure further decreases and comes to the secondcritical value 7.4 kg/cm² at, for instance, the time t12 illustrated inthe Figure, the pressure control region shifts from the region C to theregion B. At this time, since the vapor pressure is dropping down, theboiler unit #1 of the adjustment units #2 and #1 which takes precedenceover the boiler unit #2 at that time shifts from the low combustionstate L to the high combustion state H, resulting in an increment incapability of evaporation of the boiler unit #1. This makes the vaporpressure turn to increase again at around the time t12. When the vaporpressure reaches the third critical value 7.8 kg/cm² again at, e.g., thetime t13 shown in the Figure, the pressure control region changes fromthe region C to the region D. Because the boiler unit #2 of theadjustment units #2 and #1 has, as mentioned above, already been in thelow combustion state L, the boiler unit #1 in the high combustion stateH moves to the low combustion state L. As a result, the capability ofevaporation of the boiler unit #1 diminishes, and the vapor pressureturns to decrease again at around the time t13. As far as the thermalload is almost constant, the adjustment unit #2 of the boiler units #1and #2 in the combustion state is held in the low combustion state L andthe other adjustment unit #2 repeatedly shifts between the lowcombustion state L and the high combustion state H, as in theabove-described manner. Hence, the vapor pressure, as illustrated in theFigure, fluctuates between the second critical value 7.4 kg/cm² and thethird critical value 7.8 kg/cm² (t11--t12--t13--t14--t15), with theresult that the vapor pressure converges in the state holding region C.

In a case where unlike the situation at the time t10, the vapor pressuredoes not change its trend from rise to fall and still continues toincrease, even after the control region shifts, as described above, fromthe control region D to the control region E, i.e., from the lowcombustion state shifting mode to the combustion unit number decreasemode, the combustion stopping signal BD is output from the combustionunit number setting section 17 to the boiler units #1 and #2 in thecombustion state, at the time when the vapor pressure reaches apredetermined maximum critical value, for example, 8.5 kg/cm². Thisensures the combustion of these boiler units to be stopped. Such afunction can easily be achieved by modifying some portion of theconstruction depicted in FIG. 2.

The above-described control operation is based on the assumption thatthe thermal load on the boiler system concerned is substantiallyconstant. Next, a control operation when sharp fluctuations take placein the thermal load will be explained also with reference to FIGS. 2 and4. If the thermal load exerted on the boiler system sharply fluctuates,the vapor pressure sharply rises or falls. Now, it is assumed that thethermal load on the boiler system sharply increases, making the vaporpressure in the steam header 11 (FIG. 1) abruptly go down, when theboiler unit #1 shifts from the high combustion state to the lowcombustion state L at, for instance, the time t15 and the vapor pressureis falling.

(8) When the vapor pressure reaches the second critical value 7.4 kg/cm²at, for example, the time t16, the pressure control region changes fromthe region C to the region B. Since the vapor pressure is falling, theboiler unit #1 of the adjustment units #2 and #1 at that time whichprecedes in the starting order shifts from the low combustion state L tothe high combustion state H. Accordingly, the capability of evaporationof the boiler unit #1 increases, but the vapor pressure still continuesto fall. Then, at the time t17 3 sec after the time t16, the highcombustion state selecting signal BH is output from the combustion stateselecting section 18 and is supplied to another adjustment unit at thattime, viz., the boiler unit #2, which in turn shifts from the lowcombustion state L to the high combustion state H. If the vapor pressurefurther drops down and reaches the first critical value 7.2 kg/cm² at,e.g., the time t18 shown in the Figure in spite of the fact that theboiler units # 1 and #2 have come to the high combustion state, thepressure control region changes from the region B to the region A, andthe combustion starting signal BI is output from the combustion unitnumber setting section 17 and is supplied to the boiler unit #3 whichprecedes in the starting order among the boiler units #3 and #4 that arenot in operation at that time. Then, the boiler unit #3 is started inthe high combustion state H after a slight time lag. However, if thevapor pressure still remains at a level lower than the first criticalvalue 7.4 kg/cm² even at the time t19 10 sec after the time t18, thecombustion starting signal BI is output from the combustion unit numbersetting section 17 and is supplied to the last boiler unit #4 at thetime t19, making this unit #4 start in the high combustion state H. As aresult, the vapor pressure turns to rise and goes on rising to cause thecontrol region to shift from the region A to the region B (at the timet20), from the region B to the region C (at the time t21) and finally tothe region D (at the time t22). Thus the vapor pressure converges in thecontrol region C, taking almost the same course as that in theabove-stated case (from the time t10 to the time t15).

(9) Next, a case where the vapor pressure sharply increases due to adrop in thermal load will be explained. It is assumed that when theboiler unit #1 moves from the low combustion state L to the highcombustion state H at, e.g., the time t14 and the vapor pressure isincreasing, the vapor pressure, as indicated by a broken line on a vaporpressure curve of FIG. 4, sharply rises and reaches the third criticalvalue 7.8 kg/cm² at, e.g., the time t24 shown in the Figure. At thistime, the pressure control region changes from the region C to theregion D, viz., to the low combustion state shifting mode. In this lowcombustion state shifting mode, unlike the case already mentioned in theitems (4) to (7) in connection with the time t6 to the time t12, theboiler unit #3 and #4 are not in operation. The boiler unit #1 is in thehigh combustion state H, whereas the boiler unit #2 is in the lowcombustion state L. Hence, the boiler unit #1 moves from the highcombustion state H to the low combustion state L at the time t24. If thevapor pressure still continues to increase and enters the control regionE at, for example, the time t25 shown in the Figure, the boiler unit #2in the low combustion state ceases combustion in response to thecombustion stopping signal BD transmitted from the combustion unitnumber setting section 17. A difference between the number of unitswhich is set by the adjustment unit number setting switch 22 and thenumber of units which are actually being operated is produced when thecombustion of the boiler unit #2 stops. In consequence, the AND gate 19is supplied with the adjustment unit number increase request signal Bpfrom the adjustment unit number setting section 21. At this time, thevapor pressure goes on rising, and the low combustion state requestsignal BL is also input to the AND gate 19. Then, the AND gate 19outputs a high level signal. However, there is no boiler unit which isbeing operated in the high combustion state at that time, with theresult that the output signal from the AND gate 19 is ignored.

If the vapor pressure does not go out of the control region E at thetime t26 3 sec after the time t25, the boiler unit #1 in the lowcombustion state similarly ceases combustion in response to the signalBD sent from the combustion unit number setting section 17, with theresult that the entire boiler system stops its operation to cope withsuch an abnormal drop in load. Where the vapor pressure turns todecrease in the control region E after the time t25, the boiler unit #1which is solely in operation has already been in the low combustionstate L, and thus keeps its state. This boiler unit continues to holdthis state even after entering the control region C. After that thevapor pressure converges to a region in the control region C, taking thecourse mentioned in the items (6) and (7) in regard to, for example, thetime t11 to the time t14.

The embodiment of the present invention has been described above.Additions or modifications may, as a matter of course, be effected tothe embodiment of the apparatus for controlling the boiler systemaccording to the present invention. For instance, in the above-describedembodiment, it is feasible to arbitrarily modify the reference voltagevalues respectively set in the comparators 15a through 15d in thehardware construction depicted in FIG. 2 in correspondence with thecritical values of the vapor pressures between the vapor pressurecontrol regions A, B, C, D and E. The hardware construction of FIG. 3,in particular the construction of the adjustment unit number settingsection 21 may be replaced by other suitable circuit constitutions orcomputer programs. Besides, the order in which the boiler units arestarted is the same as the order in which the units are arranged in theembodiment described above. However, the starting order of therespective boiler units may arbitrarily be set regardless of thearranging order thereof.

Industrial Applicability

As discussed above, the present invention provides such an effect thatthe output of the entire boiler system smoothly converges in a desiredrange by minimizing the frequency at which the respective boiler unitsare repeatedly stopped and started. The present invention is effectivein controlling a boiler system including a plurality of boiler units.

I claim:
 1. An apparatus for controlling a boiler system, comprising:aplurality of boiler units (10) connected in common to a steam header(11) and shiftable between the halting state and the operating state inresponse to each of a combustion starting signal (BI) and a combustionstopping signal (BD) and further shiftable in said operating statebetween a high combustion state and a low combustion state in responseto each of a high combustion state selecting signal (BH) and a lowcombustion state selecting signal (BL); a pressure adjuster (12)connected to said steam header (11) for outputting a vapor pressuresignal (P) indicating the vapor pressure in said header (11); and acontrol unit (13) for controlling each of said plurality of boiler units(10) such that these boiler units takes one of three states in responseto said vapor pressure signal (P) transmitted from said pressureadjuster (12), said control unit (13) including: combustion unit numberincrease request signal generating means (15a) for outputting acombustion unit number increase request signal (Bi) by detecting thefact that said vapor pressure indicated by said vapor pressure signal(P) is equal to or smaller than a first critical value; high combustionstate request signal generating means (15b) for outputting a highcombustion state request signal (Bh) by detecting the fact that saidvapor pressure indicated by said vapor pressure signal (P) is equal toor smaller than a second critical value greater than said first criticalvalue; low combustion state request signal generating means (15c) foroutputting a low combustion state request signal (Bl) by detecting thefact that said vapor pressure indicated by said vapor pressure signal(P) is equal to or larger than a third critical value greater than saidsecond critical value; combustion unit number decrease request signalgenerating means (15d) for outputting a combustion unit number PG,28decrease request signal (Bd) by detecting the fact that said vaporpressure indicated by said vapor pressure signal (P) is equal to orlarger than a fourth critical value greater than said third criticalvalue; combustion unit number setting means (17) for sequentiallysupplying each of said plurality of boiler units (10) with saidcombustion starting signal (BI) in response to said combustion unitnumber increase request signal (Bi) according to the order of startingsaid boiler units every time a predetermined time passes, and forsequentially supplying each of said plurality of boiler units with saidcombustion stopping signal (BD) in response to said combustion unitdecrease request signal (Bd) in the order reverse to said starting orderevery time a predetermined time passes; combustion state selecting means(18) for sequentially supplying each of said plurality of boiler units(10) with a high combustion state selecting signal (BH) in response tosaid high combustion state request signal (Bh) in the order of startingsaid boiler units every time a predetermined time passes, and forsequentially supplying each of said plurality of boiler units (10) witha low combustion state selecting signal (BL) in response to said lowcombustion state request signal (Bl) in the order reverse to thestarting order every time a predetermined time passes; adjustment unitnumber setting means (21) for outputting an adjustment unit numberincrease signal (Bp) by deciding that the number of said boiler unitsoperating in the low combustion state is smaller than the number ofadjustment units on the basis of said combustion starting signal (BI)supplied from said combustion unit number setting means (17) to each ofsaid plurality of boiler units (10), said low combustion state selectingsignal (BL) supplied from said combustion state selecting means (18) toeach of said plurality of boiler units (10) and an adjustment unitnumber setting signal indicating the number of said adjustment units;and low combustion state request signal supplying means (19) forallowing said combustion state selecting means (18) to supply said lowcombustion state request signal (Bl) in response to said adjustment unitnumber increase signal (Bp).